You’ve sent in your perfectly manicured résumé and flawless cover letter. You've researched the company and gave brilliant responses to tough interview questions. You’re probably a strong candidate—but forget to smile, slouch in your chair or fail to make eye contact during the interview, and you could be out of the running.

“A candidate can give out thousands of non-verbal cues within the first minute of meeting a hiring manager, and those messages make more of an impact than the words that you use during the interview,” says Patti Wood, a body language expert and author of Snap: Making the Most of First Impressions, Body Language, and Charisma (to be released October, 2012). “Our body language says a lot about who we are and our emotional state, and poor body language often sends a message that we are stressed or fearful.”

Wood says don’t wait until you’re in the hot seat to start focusing on your body language. Be aware of your posture, your facial expressions and your gestures from the moment you arrive.

Author and career expert Dan Burns agrees. “I try to stress to people that the first impression you make happens before you even sit down to interview,” he says. “The hiring manager will look at your face, your hair, what you’re wearing and the image you are projecting, all before you have had a chance to formally meet.”

Once the interviewer greets you, make eye contact and offer a palm-to-palm handshake that is not too strong and not too weak. Keep an appropriate distance as he or she greets you. Relax your body and smile. “Don’t freeze,” Wood says. “Candidates often stiffen up when they are walking in to an interview.”

Once you’re in the hot seat, find an appropriate place to set down your belongings. Don’t put your briefcase or purse on your lap or on the table. Sit up straight, avoid touching your face and hair, and don’t cross your arms or hide your hands. “Don’t be afraid to gesture,” Wood says. Gesturing shows that you’re enthusiastic and expressive. It can also help access more information in your brain and create vocal variation, she adds.

Power and confidence are typically conveyed through body language, and so are your stress level and how open and honest you are. “An employer will get a sense of who you are and how you will perform under pressure by assessing your body language before, during and after the interview,” Wood says.

“Interview body language mistakes may tell the hiring manager that you’re flippant, scared or passive,” she adds. “If you’re under-qualified or you say the wrong thing, the interviewer can forgive that, but if your body language says you’re a person who doesn’t work well in stressful situations or that you’re not confident, that’s something they know they can’t change.”

So how do you avoid making body language mistakes? With practice and preparation.

“Practice entering and leaving a room, think about where you will put your briefcase during the interview, and plan how you will say hello and goodbye to the interviewer,” Wood says.

Preparation for the interview often builds confidence, Burns says. When you’re confident, you tend to have fewer body language issues.

The hiring manager looks for ways to set a candidate apart from others, Burns says. “The negative differentiators, like poor and ineffective body language, help make the decision easy for the hiring manager.”